Metallized film capacitors are used extensively in a broad range of electrical and electronic equipment that include motor run and motor start circuits for air conditioners, fluorescent and high intensity light ballasts, power supplies, telecommunication equipment, instrumentation, and medical electronics. In many of these applications, the metallized capacitors are used to conserve energy by correcting the power factor of a circuit and in others they are used to perform specific functions, such as timing, filtering, and decoupling. The advantages of monolithic multilayer over film foil or metallized film capacitors include lower volume, weight, cost, and higher application temperature.
High energy density capacitors that are based on polymer dielectrics are usually designed with foil electrodes and a composite insulation system that consists of polymer film, or possibly paper and a liquid impregnant. Lower voltage applications include metallized film designs which can be dry or liquid impregnated. These capacitors serve various pulsed power applications that are differentiated by the repetition rate and the dV/dt of the charge and discharge pulses. Low repetition rate applications include flashlamps, copiers, defibrillators, pulsed lasers and more recently beam weapons and electromagnetic catapults. High repetition rate conditions are common to short pulse radar modulators, isotope separation lasers, directed energy weapons and electronic warfare and countermeasure pulse generators.
When designing high energy density capacitors with materials such as polypropylene (PP), polyester (PE), polycarbonate (PC), polyvinylidene fluoride (PVDE), and paper, the limitations that the person skilled in the art is faced with when attempting to increase the energy density include:
Little or no variation in the dielectric constant and breakdown voltages due to fixed polymer chemistries; PA1 Rapid degradation of the dielectrics when the voltage of the capacitor is raised above the corona inception voltage; PA1 Loss of dielectric constant (PVDF film) when exposed to higher voltages; PA1 Dielectric degradation by thermal loads and electrostrictive forces in high dV/dt and high rep-rate pulse applications; and PA1 Dielectric thickness limitations due to mechanical strength of the self supported film. PA1 (a) the dielectric thickness between the interleaved metal electrode layers is a maximum of about 10 .mu.m; PA1 (b) the polymer is designed with a high dielectric constant, .kappa., where .kappa. is higher than 3; PA1 (c) the metal electrode layers within the polymer layer are recessed along edges orthogonal to the solder termination strips to prevent arcing between the metal electrode layers at the edges; and PA1 (d) the metal electrode resistance is within the range of about 10 to 500 ohm per square.
In order to make a significant improvement in the energy density of these capacitor systems, new films are required with high dielectric constants that are also stable at high temperatures and voltages.
Acrylate monomer films are presently formed in the vacuum with a continuous ultra high-speed deposition process and they are cross-linked using electron radiation; see, e.g., U.S. Pat. Nos. 5,018,048 and 5,097,800. These films have excellent thermal and mechanical properties, and high-quality polymer monolithic (PML) chip capacitors with dielectric constant, .kappa., equal to 3.5 have been produced for electronic applications. These capacitors are limited to low voltage applications (&lt;50V). Since the maximum capacitor energy density D.sub.m is equal to EQU D.sub.m =1/2.kappa.e.sub.o E.sub.m.sup.2 (J/cm.sup.3) (1)
where .kappa. is the dielectric constant, e.sub.o is the permittivity of free space, and E.sub.m =V/t=maximum field, where V is the maximum applied voltage and t is the dielectric thickness, then the PML capacitors have limited energy density capabilities.
What is needed is a thin film capacitor that has a material with comparatively higher dielectric constant that can also withstand high applied voltages.